The higher charge load may be making the brass not shrink away from the chamber mouth adequately before the bolt unlocks and extraction begins. That in effect delays the cycling a little bit and possibly enough to lose the momentum needed for the bolt to travel all the way back. IE not the gas system's fault. Its trying its heart out to cycle but the case is still holding on a little. Usually however, you see pressure signs in the form of case head growth when precisely measured. See if those problematic rounds cases will slide back into the chamber using your finger to push them back in. If they don't or get real snug, you might have subjected them to excessive pressure. Clean the bore and the wipe down any film on the outside of the cases with some mineral spirits before trying to reinsert them, to make sure all you're feeling is resistance (if any) due to case head growth. I sort my brass primarily by the fattest diameter I can measure with a micrometer. That usually occurs just above the web. My rifle will not chamber rounds in excess of .5017, .5000 being SAAMI spec for the caliber at that point.

Hoot

Edit: In case you're wondering, resizing the brass will not remove case head growth.

In Theory, there is no difference between theory and practice. In Practice, there is.

As all my cases normally go into a Fired Case bin upon firing I had no way of checking the diameter of problematic case. Thus thirty rounds were loaded as follows:

IMR 4227 : Five rounds each with 39.0, 40.0, 41.0 and 42.0 grs of powder.

H-110 : Five round each with 41.0 and 44.0 grs of powder.

Hornady Factory : An additional ten rounds to serve as reference.

Firing of the factory and H-110 rounds went without a hitch. However the IMR-4227 40.0 and 42.0 gr loads had a total of five cycle failures. The IMR 4227 39.0 and 41.0 loads worked fine.

The diameter of all cases were measured and found to be .498 to .500. Sorry my micrometer only measures to the thousandths of an inch. There was no noticeable difference between the groups of ammo.

After pondering this for awhile I checked the relative burn rate table to see if there were any other powders worth trying and noticed that IMR-4227 is the slowest powder listed for the 450 Bushmaster. That is one of the reasons for checking out H-110 which has similar performance to IMR-4227. H-110 will now be use in an attempt to find a good load for my gun.

My original attempts at loading the 450 were with Lil’Gun which worked fine but that powder made the gun extremely hot. An example is shooting my Ruger single seven with a 100gr bullet and Lil’Gun. After the third cylinder full, fired in normal slow fire, the gun was so hot I could not hold it except by the grip. That's why I don’t use Lil’Gun anymore.

As a side line, up to a couple days ago the scope used was a second hand 1-4X which had problems with focusing. I broke down and purchased a Vortex 1.75-5 x 32 Dimondback which was used on these tests. The target was only at 50 feet but seeing five round go into a 5/8” hold is very comforting.

I did not post for awhile because I was waiting for a Stealth Recoil Spring from Armaspeg. It arrived last Friday and when attempting to install it I realized it was for a carbine stock not a rifle stock. The spacer to accommodate the difference was ordered and arrived today.

When it was installed the action was very stiff and difficult to manually chamber a round. A Lancer magazine was loaded with three Hornady factory rounds and two with a 230 gr bullet and 41.0 grs of H-110. None of the rounds cycled properly.

Next the original buffer and spring were reinstalled and the Lancer magazine loaded with eight rounds of 230 gr bullets and 41.0 grs of H-110. They all fired with on problems.

I have concluded that the Stealth Recoil Spring is a dismal failure.

In the future I will be using H-110 for developing the loads for this rifle. My powder measure handles the powder very nicely and it fills the case to about 80 or 90 percent which I like.

Also I am very satisfied with the Lancer 20 round magazine for this rifle. I think eight rounds is more than enough for this rifle. It was enough for my M1 service rifle.